1. Field of the Invention
The present invention relates to a cooling system for four-stroke cycle internal combustion engines, and more particularly to a cooling system for cooling a cylinder head portion of a four-stroke cycle internal combustion engine.
2. Description of the Related Art
An air cooling type four-stroke cycle internal combustion engine used in a portable trimmer, a chain saw or the like is subject to overheating around the combustion chamber or spark plug. There have been known various engine cooling systems for preventing such overheating. For example, Japanese Patent No. 3168140 discloses an engine cooling system in which an engine body having a spark plug screwed in the top wall of each cylinder is covered by a shroud to define a cooling air channel between the engine body and the shroud, and a rotor having a cooling blade is fixed to the outer end of a crankshaft supported by the engine body, so that the cooling blade is rotated by the crankshaft to generate a cooling airstream in the cooling air channel. In this cooling system, the engine body is also formed with a primary air channel for leading the cooling air from the cooling air channel through a valve chamber in the engine body and the space between intake, and exhaust ports in the engine body toward the spark plug, and an auxiliary air guide channel intersecting the primary air channel to allow the cooling air introduced from the cooling air channel through inlets at both ends of the primary air channel to be led to the primary air channel. Such a cooling system can prevent overheating from occurring around the combustion chamber or spark plug.
Another known four-stroke cycle internal combustion engine is one in which a lubricating oil reserved in an oil reservoir chamber is atomized to form an oil mist and a valve mechanism in a valve chamber is lubricated by the oil mist. In this type of four-stroke cycle internal combustion engine, overheating of the oil mist leads to insufficient lubrication and excessively increased oil consumption.
It is therefore an object of the present invention to provide a four-stroke cycle internal combustion engine cooling system capable of preventing overheating of oil mist for lubrication, particularly for an OHC type four-stroke cycle internal combustion engine in which the oil mist flowing around a valve chamber is apt to receive excessive heat because the valve chamber is located above a combustion camber.
In order to achieve the above object, according to the present invention, there is provided a cooling system for a four-stroke cycle internal combustion engine in which a lubricating oil reserved in an oil reservoir chamber is atomized to generate an oil mist and a valve chamber is lubricated thereby. This cooling system comprises an accommodating space which accommodates a transmission mechanism for transmitting the rotational movement of a crankshaft to a camshaft in the valve chamber and extends vertically along the side of a cylinder block, a blower fan rotatably driven by the crankshaft to send an air upward through the accommodating space, and a cooling air channel extending laterally between a combustion chamber in the cylinder block and the valve chamber disposed above the combustion chamber to receive the air from the accommodating space. The cooling air channel includes a primary air channel formed between intake and exhaust ports extending laterally in respective opposite directions. The primary air channel extends toward a spark plug located on the downstream side of the cooling air channel. The cooling air channel also includes an air guide channel for guiding the air from a pair of air inlets opposed to one another to an upstream inlet of the primary air channel. The air guide channel is defined by a wall which faces the upstream inlet of the primary air channel and extends laterally along the intake and exhaust ports. The wall extends vertically in the accommodating space. Further, the wall is formed with an oil mist passage extending vertically therein to supply the oil mist from the oil reservoir chamber to the valve chamber.
In the present invention, when the blower fan is rotated by the rotation of the crankshaft, the generated airstream reaches the primary air channel through the accommodating space and the opposed air inlets of the air guide channel.
According to the present invention, the vicinity of the spark plug is cooled by the air from the cooling air channel. Since the cooling air channel is disposed between the valve chamber and the spark plug, the valve chamber hardly receives any heat from the vicinity of the spark plug. The bottom wall of the valve chamber is also cooled by the air flowing through the cooling air channel. Thus, the valve chamber is prevented from being excessively heated. Further, since the oil mist passage is formed in the wall extending vertically in the accommodating space, the oil mist flowing through the oil mist passage is cooled by the airstream in the accommodating space. In this manner, the valve chamber and the oil mist passage are adequately cooled. This prevents insufficient lubrication and excessive oil consumption.
In one embodiment of the present invention, the cylinder block may be formed with a plurality of air-cooling fins extending outward from the periphery of the cylinder block. In this case, the wall having the oil mist passage extends vertically through the plurality of air-cooling fins.
In another embodiment of the present invention, the wall may have a laterally extended cross-section, and the oil mist passage may comprise a pair of circulation passages extending vertically in respective side portions of the wall. In this case, the circulation passages are in fluid communication with the oil reservoir chamber and the valve chamber to allow the oil mist to be circulated from the oil reservoir chamber to the valve chamber and from the valve chamber to the oil reservoir chamber. Each of the circular passages has a substantially circular cross-section, and each periphery of the side portions of the wall is curved along the cross-sectional shape of the corresponding circular passage to enlarge the corresponding air inlet of the air guide channel in cross-section.
In still another embodiment of the present invention, the wall may include a concave portion provided in its surface facing to the accommodating space between the circulation passages so that the wall has a substantially C-shaped cross-section opened toward the accommodating space.
In yet another embodiment of the present invention, the primary air channel and the air guide channel having the pair of opposed air inlets may be combined to form a substantially T-shaped cooling air channel or a substantially Y-shaped cooling air channel.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description.